The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the treatment of oil and gas wells, such as hydraulic fracturing treatments, viscosifiers, such as polymer systems, are commonly used in carrier fluids. A fluid loss additive (FLA) is often used with such carrier fluids to inhibit excessive fluid loss from the carrier fluid. The FLA helps form a filter cake on the surface of the formation. In a fracturing operation, the fluid efficiency is directly related to the amount of fluid loss. High fluid efficiency minimizes the amount of fluid needed to generate a given length of fracture and limits the amount of filter cake that is generated. Fluid loss additives can be used to decrease fluid loss and increase the fluid efficiency. The filter cake formed by the FLA reduces permeability at the fluid-rock interface. Conventional FLA usually consists of fine particles, such as mica or silica flour with a broad distribution of particle sizes designed to effectively plug the pore throats of the rock matrix. Starches or other polymers can be added to help fill in the spaces and further reduce the flow.
The FLA is usually injected into the fracture with the initial pad volume used to initiate hydraulic fracturing. After the pad is injected, proppant slurry, which may also contain an FLA, is pumped into the fracture in various stages depending on job design. The proppant is designed to hold the fracture open and allow reservoir fluid to flow through the proppant pack. The proppant slurry generally includes a viscous carrier fluid to keep the proppant from prematurely dropping out of the slurry. After the proppant has been placed in the fracture, the pressure is released and the fracture closes on the proppant. After the treatment, it is necessary to remove or break both the viscosifier in the carrier fluid and the filter cake (that may contain viscosifier polymer) so that reservoir fluids can thereafter flow into the fracture and through the proppant pack to the wellbore and the production string.
Fracture clean-up issues are a known problem. Although other systems such as viscoelastic surfactants, gelled oil, slick water, etc. are used, the majority of fluids used to create the fracture and carry the proppants are polymer-based. In most reservoirs with lower permeability, the polymer concentrates as carrier fluid leaks off during the fracturing process. The concentrated polymer hinders fluid flow in the fracture and often results in underperforming fractures. Typical remedies include use of breakers, including encapsulated breakers that allow a significant increase of the breaker loading. The breaker is added to the fluid/slurry and is intended to reduce the viscosity of the polymer-based carrier fluid and facilitate fracture clean-up. Despite high breaker loading, in such breaker systems the retained permeability of the proppant pack is still only a fraction of the initial permeability and this has been the accepted situation in the industry. In certain cases, the encapsulated breakers may have a large particle size (e.g. 1 mm) that prevent the solid breaker material from entering small natural fractures of the formation so that the polymer that enters these small fractures remains unbroken. And soluble breaker materials are only used in limited concentrations as the base fluid rheology must be maintained for some time. If too much soluble breaker is used, the viscosity of the fluid may drop prematurely. Soluble breaker materials may also tend to leak off into the formation, where they are no longer effective.
U.S. Pat. No. 7,337,839 discusses fracture clean-up issues and the mechanisms of polymer concentration and the role played by the filter cake. Contrary to the prevailing theory and the industry-accepted practice of modeling polymer concentration as an average involving all the fluid pumped, the filter cake can be the main and only medium where significant polymer concentration takes place. Embodiments of the present invention provide further approaches and methods to improve the clean-up of hydraulic fracturing treatments.